Remove preview-mode.md

This whole concept has moved somewhere else,
and all information from that file has been retrieved.
So it can safely be removed.
display-slur-control-points: Make appearance configurable

Replace hard-coded values in the calling lilypond function
by variables.
Conditionally define these variables (can be defined individually
in the input files).
Make line thickness be configurable individually
(instead of using (/ cross-line-thickness 4).

TODO:
- Check syntax of (if defined?) constructs
- Make color configurable too (check ly:stencil-in-color)